1. Field of the Invention
The present invention relates to motorized power equipment and more specifically to motorized power equipment having an improved hydraulic pressure bypass mechanism.
2. Description of Related Art
Hydrostatic transmissions are typically used in motorized power equipment to provide movement. The primary function of any hydrostatic transmission (HST) is to accept rotary power from a prime mover (usually an internal combustion engine) having specific operating characteristics and transmit that energy to a load having its own operating characteristics. In the process, the HST generally must regulate speed and direction of rotation. Torque and power are generally constant because pressure is constant regardless of volumetric flow which regulates speed. Depending on its configuration, the HST can drive a load from full speed in one direction to full speed in the opposite direction, with infinite variation of speed between the two maximums, all with the engine operating at constant speed.
The operating principle of HSTs is relatively straight forward; a pump, connected to the engine, generates hydraulic fluid flow and pressure to drive a hydraulic motor, which is connected to the load. If the displacement of the pump and motor are fixed, the HST simply acts as a gearbox to transmit power from the engine to the load. Most HSTs use a variable-displacement pump so that speed and direction can be regulated. Some advantages of HSTs include, but are not limited to, the ability to transmit power in a relatively compact size and operate efficiently over a wide range of speed ratios at constant torque.
There are typically two types of HSTs, integrated and non-integrated (separate pumps and motors). The integrated HST consists of an integrated motor and pump together in the same housing. In a non-integrated construction the pump and motor are located separately and are coupled together via hoses and tubes. In this construction the engine is connected to the pump and the motor is connected to the load. By separating the pump and motor, multiple motors may be connected to the pump, thus allowing a single engine to operate multiple pumps. One advantage of this type of HST is that it allows power to be applied to loads that might be in locations that may be difficult to access.
There are two basic integrated HST configurations; open-circuit and closed-circuit configurations. Both refer to how the hydraulic lines in the system are connected. In an open circuit, fluid is drawn into the pump through a reservoir, routed to the motor, then re-enters the reservoir after passing through the hydraulic motor. In a closed circuit, the flow path is uninterrupted—fluid flows in a continuous path from the pump discharge port to the fluid motor inlet port, out the motor discharge port and back into the pump inlet.
When the HST is in operation, the flow of fluid within the HST causes the motor to rotate a drive shaft which in turn rotates a wheel (or wheels) connected thereto. The rotational speed of the wheel may be controlled by various gears or belts depending on how the drive shaft is connected to the wheel. However, when fluid is not flowing within the HST, the residual pressure causes the drive shaft to remain stationary and may be locked into place. When locked into place by the HST, the drive shaft may also cause the wheel or wheels to be locked into place.
In order to alleviate the locked condition, a pressure release valve may be utilized. The pressure release valve may cause the hydraulic pressure within the HST to be released as outlined in U.S. Pat. No. 6,755,019 issued to Phanco. As described in Phanco, rotating a bypass arm actuator causes a bypass actuator to separate the hydraulic motor from its motor running surface, thus releasing the pressure and allowing the wheels connected to the HST to freely rotate or “free wheel”. Although, some resistance will still be present from fluid flow and mechanical friction.
The prior art bypass mechanism as described in Phanco uses rotational energy in order to activate the bypass mechanism. In some HST's the ability to rotate the bypass mechanism may be difficult given the tight spaces the HST's may be mounted into. The present invention addresses the differences as described in Phanco by providing a bypass mechanism that is pushed or pulled instead of rotated. Pushing or pulling the bypass mechanism instead of rotating it may allow the location of the bypass mechanism to be positioned on a part of the HST which may have greater clearance.